This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. SUMO. The small ubiquitin-like modifier SUMO regulates nuclear transport, stress response, and signal transduction in eukaryotes, a process that is essential for cell cycle progression in yeast. Analogous to ubiquitin modification, SUMO conjugation occurs on lysine residues and is catalyzed by E1, the SUMO activating enzyme, E2, the SUMO conjugation enzyme, E3-like conjugation cofactors, and proteases that catalyze SUMO processing and deconjugation. SUMO modification does not typically target proteins for degradation, but rather alters the target protein function through changes in cellular localization, biochemical activation, or through protection from ubiquitin-dependent degradation. We have structurally characterized components of this system, both alone and in complex with substrates and co-factors. We are continuing efforts to characterize the structural basis for additional complexes between E1, E2, E3, SUMO with various substrates and cofactors. mRNA maturation and decay. The stability and lifetime of cellular mRNA depends on pre-mRNA processing and decay pathways. We study early processing events associated with RNA capping including the catalytic steps that are required in eukaryotic organisms to cap the nascent mRNA chain. Each of the cap-forming activities is essential for cell growth and we are characterizing the structural basis for several of these enzymes in complex with each other, in complex with RNA and oligonucleotide compounds, and in complex with phosphorylated CTD from RNA polymerase II. RNA decay also plays an important role in RNA metabolism, and we are engaged in studies aimed at elucidating regulatory mechanisms that regulate 3'-5'decay as catalyzed by the eukaryotic exosome, a large multi-subunit complex that contains subunits that catalyze processive and/or distributive 3'-5'exoribonuclease activities.